A servomotor having a high resolution encoder, which is used in a conventional machine tool or the like, is disclosed in Japanese Patent Laid-Open No. HEI 2-188145. FIG. 1 of the attached drawings is a longitudinal cross-sectional view showing a coupling mounting type in which a servomotor is fixedly mounted to the outside of a bracket by a coupling. In FIG. 1, a rotor 1 has a rotary shaft 2 to which a plurality of permanent magnets 3 are fixedly mounted. A stator 4 has a fixed iron core 5 about which a coil 6 is wound. The rotor 1 is supported by a first bracket 7 and a second bracket 8 by their respective first and second bearings 9 and 10.
In the example illustrated in FIG. 1, a detector 11 uses an optical encoder. An outer frame 12 is fixedly mounted to the second bracket 8. The reference numeral 13 denotes a cover. An encoder shaft 14 is rotatably supported by a pair of bearings 15 and 16. A detecting scale 17 is fixedly mounted to the encoder shaft 14. Generally, the detecting scale 17 is manufactured such that chrome is applied to a glass material, and slits of a requisite pattern are formed in the glass material by etching. A stationary scale 18 is formed similarly to the detecting scale 17. The stationary scale 18 is provided with a requisite pattern, and is fixed such that a gap of 30.about.50 .mu.m is set between the stationary and detecting scales 18 and 17 in order to largely vary or change an output from the stationary scale 18 as compared with that from the detecting scale 17.
In the case where the encoder is an incremental type, the aforesaid patterns are arranged such that, in order to detect the rotational angle, the detecting scale 17 and the fixed scale 18 output two signals including sine and cosine having requisite resolution (the number of slits) and shifted in phase electrically by 90 degrees, a single signal of a rotational single pulse serving as a rotational-angle standard, and three.about.four signals for detecting a position of the permanent magnet 3. Further, in the case where the encoder is an absolute-value type, the aforementioned pattern is an absolute-value code (gray code or the like) having requisite resolution. However, an output is divided by an electrical interpolation to improve resolution.
The reference numeral 19 denotes a light emitting element, and an LED (light emitting diode) is generally used as the light emitting element. The reference numeral 20 designates a plurality of light receiving elements. The reference numeral 21 denotes a printed circuit board on which a signal processing circuit 22 is mounted. The reference numeral 23 denotes a flexible coupling; 24, a retainer or a presser plate for axially fixing the first bearing 9; and 25, a preload spring for applying axial pressure to the rotor 1.
FIG. 2 is a longitudinal cross-sectional view showing a conventional system in which a motor is directly mounted to the outside of the second bracket 8 and which does not use a coupling. In FIG. 2, the reference numerals identical with those used in FIG. 1 indicate the same or equivalent parts or elements. A boss 26 is fixedly mounted to the rotary shaft 2 by a nut 63. The detecting scale 17 is fixedly mounted to the boss 26. A mounting frame 27 is in the form of a reversed letter C, and the light emitting element 19 and the light receiving element 20 are fixedly mounted to the mounting frame 27. The mounting frame 27 is mounted to the printed circuit board 21 which is fixedly mounted to a cylindrical projection 8a on the second bracket 8.
FIG. 3 is a longitudinal cross-sectional view showing an encoder section of the conventional coupling mounting type. The encoder section utilizes an optical fiber and is disclosed in Japanese Patent Laid-Open No. HEI 2-10113. In FIG. 3, the reference numerals identical with those used in FIG. 1 indicate the same or equivalent parts or elements. Optical fibers 28 having their respective cores have their respective one ends 28a which face toward the light emitting element 9. The other ends 28b of the respective optical fibers 28 face toward the light receiving element 20 through the pattern of the detecting scale 17 and the pattern of the fixed scale 18.
FIG. 4 shows, of the conventional signal processing circuit 22, a section in which an output from the light receiving element 20 is divided by an electric interpolation to raise resolution. The reference numeral 20a denotes a light receiving element which outputs sine; 20b, a light receiving element which outputs cosine; 29a and 29b, amplifier circuits, respectively; 30a and 30b, A/D converters, respectively; and 31, a memory circuit which is formed by ROM (read-only memory). A conventional example of this kind is disclosed in Japanese Patent Laid-Open No. SHO 58-102110, Japanese Patent Laid-Open No. HEI 1-321313, and the like.
The above-described conventional example is directed to the servomotor including the encoder having high resolution. In the case where the servomotor is one which is low in resolution, there is a conventional example of type in which an encoder illustrated in FIG. 5 is incorporated in the servomotor. The conventional example is disclosed in Japanese Utility Model Laid-Open No. SHO 63-88075. In FIG. 5, parts or elements like or similar to those shown in FIG. 1 are designated by the same or like reference numerals. The detecting scale 17 in the form of a cylindrical metal cup which is fixedly mounted to the permanent magnet 3, has an outer periphery 17b of the cylindrical cup which is formed therein with slits 17c extending in the axial direction. Because of the low resolution, the stationary scale 18 is not required or can be dispensed with, and a gap equal to or more than 100 .mu.m is defined between the detecting scale 17 and the light receiving element 20.
The operation will next be described. When the light emitting element 19 is turned on, a slit pattern of the detecting scale 17 is irradiated. In FIG. 3, a light is incident upon one of the ends 28a of the optical fibers 28, passes through the interiors of the respective optical fibers 28, and is emitted from the other ends 28b, to irradiate the slit pattern of the detecting scale 17. At this time, when the rotary shaft 2 rotates, the detecting scale 17 also rotates. Accordingly, the light from the light emitting element 19 repeats transmission/interception by the slits formed in the detecting scale 17. The transmitted light further passes through the fixed scale 18 to irradiate the light receiving element 20. The light receiving element 20 converts the requisite signal to an electrical signal in accordance with the slit pattern, and outputs the converted electrical signal.
At this time, since a sine wave signal is outputted from the light receiving element 20a and a cosine wave signal is outputted from the light receiving element 20b, detection of the rotational angle is amplified by the amplifier circuits 29a and 29b in the signal processing circuit 22, and is converted to digital values by the respective A/D converter circuits 30a and 30b with bits in which requisite resolution is obtained. The digital values are inputted to the memory circuit 31 as address signals.
Since rotational-angle information (which is fixed or constant rotational-angle information regardless of the mounted servomotor) corresponding to the address signals is stored beforehand in the memory circuit 31, the rotational-angle information corresponding to the address signals is outputted from the memory circuit 31. Accordingly, the outputs from the respective light receiving elements 20a and 20b are interpolated electrically so that the resolution can be improved.
Since the conventional servomotor of the coupling mounting type requires the coupling and the bearings, one problem is that an axial dimension or size is lengthened extremely.
Further, although the non-coupling type in which the motor is mounted directly to the outside of the second bracket 8, is slightly shortened as compared with the coupling mounting type, the problem of excessive length still remains. Also, since the gap between the detecting scale and the stationary scale must be reduced, a further problem in the non-coupling type is that the machining accuracy of the rotary shaft and the first and second brackets must be high.
Moreover, the detecting scale can be formed therein only with a finite number of slits, and utilizes an electrical interpolation in order to improve the resolution. Since, however, the rotational-angle information is not inputted in accordance with runout of the rotary shaft due to rotation of the servomotor, fluctuation in the relative relationship between the detecting scale and the stationary scale causes the output from the light receiving element to be varied so that interpolation accuracy is deteriorated, and an error occurs in the rotational-angle information. As a result, there is a limit in the resolution.
Further, since the encoder is mounted to the outside of the second bracket in the non-coupling type, in the case where the second bearing must be replaced with a new one on the basis of maintenance and inspection of the servomotor, the encoder section must be disassembled and dismounted. Thus, there is a further problem that, at assembling, the encoder section must be readjusted mechanically and electrically.
Furthermore, regarding the second bearing, since steps to prevent scattering of enclosed grease are not taken, the slits in the detecting scale may become contaminated.
Moreover, the type in which the encoder is incorporated therein has the following problem. That is, reliability of the rotational-angle information is reduced by the limited machining accuracy with which the rotary shaft of the servomotor and the first and second brackets are made, by the thermal expansion of the constitutional mechanical parts or elements of the detecting scale or the like due to heat generation of the servomotor, and by the limited heat resistance of the signal processing circuit. Thus, based on these problems, it is difficult to improve the resolution.